for those folks interested in the Pandora's Box 4 or the King of the Air vertical 51 in 1 .I've made a few different videos testing out both the Pandoras Box 4 - 645 in 1 and the King of Air 51 in 1 vertical PCBs. check them out.

The good:
I like that it's small, boots every time without issues and is a nice quick test board.
Some of the emulation is great, others not so great. Its a 'cheap' quick fix way to get a lot of games in a single cab conveniently.

The bad:
If you know certain games well from the real hardware, you will notice that they are using old mame emulation / drivers, and will notice oddities in audio quality, scrolling on some, or things like flickering sprites.

At worst, the box doesn't retain any highscores at power-off, and dealing with credits / exiting from games, the loading of games audio and bootup videos can be brutally annoying. Vertical games are mushed into a central column on the 4:3 screen with no option to rotate. Very little flexibility to change dip options or much else on a per-game levels. VGA out works, but looks terrible in my view (mame x2 smoothing filter) and I think games run even more slowly in this mode. There's also no easy way to get stereo out for say Neo Geo titles - not on my version of the box anyway.
There is space on the circuit board for a headphone socket, I've just not managed to get anything working on it from probing / testing.

And that's an added thing - you don't really know what exact version you will be getting. It could be a 'bootleg' or more of a bootleg than the 'originals'.
I've seen some with images of headphone sockets on the side.

There are about 195 versions of Street Fighter, Art of Fighting, and King of Fighters including bootlegs. Fine if you like fighting games or the Asia slanted arcade scene. Not so much if you like titles that were more popular in the west.
Some of my faves like Pang, Rainbow Islands, Rod-Land, Sky-Kid, Ordyne, Alien Syndrome, Escape From the Planet Of Robot Monsters, Rolling Thunder, Liquid Kids, Hammerin' Harry, Mr Driller, Puyo Puyo, Smash-TV, Sly Spy, Choplifter, PacMania, or racers like Outrun, Road Blasters, and ChaseHQ... all missing. But they have included odd Korean bootlegs of Tumble-Pop and Snow Bros.
Maybe controls proved to be an issue for some titles like racers.. and games like Ikari Warriors or Midnight Resistance. That's fine and to be expected. Yet Arkanoid is on there, with joystick controls.. but Revenge of Doh isn't.

Ultimately though a decent MAME PC will eclipse the Pandora Boxes until their emulation gets a lot better / more powerful, and a raspberry pi solution will give you more flexibility in terms of editing games lists and how you want stuff emulated.
Sellers of the Pandora boxes have dipped onto the forums occasionally, but here have been no replies to requests about how to edit the games on the 16Gb SD card that's on the board.